The invention relates a method for dewatering of biological sludge. It comprises addition of a flocculant to a biological sludge, which comprises an aqueous phase and suspended solid organic material, flocculating and dewatering the sludge. The flocculant comprises a polymer composition, which comprises a cationic crosslinked first polymer, which is selected from crosslinked polyamines, and a cationic second polymer, which is a polymer obtained by polymerisation of (meth)acrylamide and cationic monomers, the second cationic polymer being polymerised in presence of the cationic first polymer.